The present invention relates to a negative electrode of a non-aqueous electrolyte secondary battery (hereinafter, referred to as xe2x80x9csecondary batteryxe2x80x9d) whose electrochemical properties such as the charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in negative electrode materials. The present invention further relates to a secondary battery using the same.
In recent years, lithium secondary batteries using non-aqueous electrolytes, which are used in such fields as mobile communications devices including portable information terminals and portable electronic devices, main power sources of portable electronic devices, small domestic electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density.
When a high-capacity lithium metal is used as a negative electrode material of a secondary battery, dendritic deposits of metal are formed on the negative electrode during charging. Over repeated charge/discharge cycles, these dendritic metal deposits penetrate through separators to the positive electrode, causing an internal short circuit. Furthermore, reactivity of the deposited lithium is high since they have a large specific surface area. Therefore, the dendrites react with solvents in the electrolytic solution on their surfaces, and form a surface film similar to a solid electrolyte which has no electronic conductivity. This raises the internal resistance of the batteries, causing some particles to be excluded from the network of the electronic conduction, thereby lowering the charge/discharge efficiency of the battery. For these reasons, the lithium secondary batteries using lithium metal for a negative electrode material suffer a low reliability and a short cycle life.
Nowadays, lithium secondary batteries which use carbon materials capable of intercalating and deintercalating lithium ions, are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, short circuits are not caused by dendrites. However, the theoretical capacity of graphite which is one of the currently available carbon materials, is 372 mAh/g, only one tenth of that of pure Li metal.
Other known negative electrode materials include metallic materials and non-metallic materials which form composites with lithium. For example, the composition formula of compounds of silicon (Si) with the maximum amount of lithium is Li22Si5. Within the range of this composition formula, metallic lithium does not normally deposit to form dendrites. Thus, an internal short circuit due to dendrite formation does not occur. Furthermore, the electrochemical capacity between the compound and each element is 4199 mAh/g, larger than the theoretical capacity of graphite.
As other compound negative electrode materials, the Japanese Patent Laid Open Publication No. H07-240201 discloses a non-iron metal siliside comprising transition elements. The Japanese Patent Laid Open Publication No. H09-63651 discloses a siliside with a fluorite structure.
Although many materials enjoy a capacity higher than carbon material, they have the following problems.
The charge/discharge cycle properties of the negative electrode material comprising pure silicon are inferior to carbon negative electrode materials. The reason for this is assumed to be the destruction of the negative electrode materials caused by their volume expansion and shrinkage.
On the other hand, non-iron metal silisides comprising transition elements and fluorite structure silisides have been proposed by the Japanese Patent Laid Open Publication No. H07-240201 and the Japanese Patent Laid Open Publication No. H09-63651, as a negative electrode material with improved life cycle properties. But, they have following problems.
The capacities of the battery using non-iron metal silisides comprising transition elements as a negative electrode material is estimated from examples and a comparative example. At the first cycle, the fiftieth cycle and the hundredth cycle the batteries of the invention have improved charge/discharge cycle properties compared with lithium metal negative electrode materials. However, when compared with a negative electrode material of natural graphite, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid Open Publication No. H09-63651 have a better charge/discharge cycle property than a Lixe2x80x94Pb alloy negative electrode material according to an example and a comparative example. The materials also have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly up to the 10xcx9c20th charge/discharge cycles. The discharge capacity lowers to approximately 70% or less of the initial capacity after about the 20th cycle. Thus, their charge/discharge properties are inferior.
Considering the foregoing problems, the inventors have proposed a negative electrode material in which particle of pure silicon or silicon compounds, a high capacity material (hereinafter, nucleus particle) is coated with a layer of a solid solution or an intermetallic compound comprising silicon (hereinafter, coating layer), in the Japanese Patent Laid Open Publication No. 2000-30703. In the case of this negative electrode material, changes in volume occurring during charge and discharge of the nucleus particle are absorbed and moderated by the coating layer. Thus, compared with the negative electrode material using only pure silicon or silicon compounds, the expansion of the material is drastically reduced.
However, the nucleus particle of the negative electrode material is not always completely covered with the coating layer. In some cases, part of the nucleus particle, for example, a pure silicon is exposed to the surface of the negative electrode material. As the electronic conductivity of pure silicon is very low compared with the coating layer, the electronic conductivity of the particle as a whole becomes low. This further worsens when the number of particles which do not contribute to the electrochemical reaction is increased by the pulverization caused by repeated charge/discharge cycles.
Considering the aforementioned problems, the present invention aims at providing a high-capacity secondary battery with improved charge/discharge cycle properties, and a negative electrode for such a secondary battery by using the new high-capacity negative electrode material mentioned above, and by improving their electronic conductivity.
The negative electrode for the non-aqueous electrolyte secondary battery is constructed such that; part of or the entire surface of a particle including at least silicon is coated with;
a) a solid solution composed of silicon and at least one element selected from a group comprising group 2 elements, transition elements, group 12 elements, group 13 elements and group 14 elements exclusive of carbon and silicon of the Periodic Table; or
b) an intermetallic compound composed of silicon and at least one element selected from a group comprising group 2 elements,-transition elements, group 12 elements, group 13 elements and group 14 elements exclusive of carbon and silicon of the Periodic Table. Part of or the entire surface of the particle coated with the solid solution or intermetallic compounds is coated with a conductive material.
This construction allows the negative electrode material to enjoy an improved electronic conductivity so that the charge/discharge cycle properties of the secondary battery are improved.